Every night, Lisa Dwan surrounds her mouth with thick black make-up, clamps her blind-folded head into a vice fixed eight foot above the stage and begins to speak at a ferocious pace.

For the next eight and a half minutes, audiences plunged into darkness can see only her spotlit, disembodied mouth expelling Samuel Beckett’s looping, babbling monologue Not I “at the speed of thought”.

It’s been dubbed the single hardest part in acting – Beckett’s muse Billie Whitelaw described it as “falling backwards into hell” – but the Hampstead-based Irish actress pulls off a coup de theatre that is part installation, part existential howl and part hallucination.

“One of the components of this installation is the actor’s terror. It’s written in such a way that no matter how many years you do it, there’s not a hope in hell you can become complacent for even two seconds,” says Dwan.

“The moment I’m strapped into that harness there’s immediate and extreme terror. You are battling your internal Not I. If you relax for one second your own demons come out to play.”

The 36-year-old has been “extremely passionate” about Beckett since working on a film festival in Dublin aged 18.

“An actor told me about this play that because of the sensory deprivation in the theatre, the mouth appears to move and the audience experiences a collective hallucination.

“He forgot to tell me, or maybe I wiped it from my memory, that several actresses had gone mad trying to learn it!”

Not I’s first UK performance at the Royal Court in 1973 starred Whitelaw and was directed by Beckett himself who counselled against ‘too much colour’ – which she interpreted as ‘don’t act’.

When Dwan read it, she was amazed to find “a transcript of how my mind works, not as a linear stream of consciousness but a heap of interrupted thoughts, rhythms and other voices filtering in from outside.”

For her first performance in 2005, Dwan put all thoughts of Whitelaw’s performance out of mind to establish her own interpretation.

But when the pair later met, they “greeted each other like two long lost war veterans” and Whitelaw called to offer Beckett’s production notes.

“I went round expecting her to whip out an old manuscript but she sat me down at her kitchen table and said: ‘Begin’.

“Then she started conducting me as Beckett had done with her, across that very table.”

Rapid pace

Jessica Tandy’s New York performance, which Beckett hated, lasted 22 minutes, Whitelaw’s 14, and Dwan, by dint of circular breathing and not swallowing was down to 12, with an instinct to push harder.

“I was afraid I was going too fast but she said, ‘You can’t go fast enough’. Beckett wanted this at the speed of thought and although every word has to be heard and felt, he wasn’t concerned with intelligibility. He wanted it to bypass the intellect. It has to play on the nerves of the audience by going at a speed the human mind can’t grasp intellectually.”

Now at eight and a half minutes, Not I runs with two other late Beckett pieces, Footfalls, in which an ethereal figure paces rhythmically outside a room, and Rockaby, which features Dwan upright in a chair that apparently rocks itself, speaking about death.

The trio, lasting only 55 minutes, have been hailed by critics as intense, beautiful, and disturbing and transfer to the West End after a sell out run at the Royal Court.

A former ballet dancer, Dwan was able to co-ordinate the physical and verbal rhythms of footfall. “Beckett is like a composer, I embrace his musicality. The original manuscript is written like a musical score.”

As Beckett grew older, he distilled his theatrical vision into minimalist, abstract pieces, which can scare audiences.

“People meet late Beckett with a type of suspicion they don’t bring to other art forms. They feel they have to crank up their intellect.

“Earlier work like Waiting For Godot is more verbose, but his late work is a purer and leaner exploration of existence, of what it is to be.

“All of Beckett’s plays, however short, are full act plays, there is enough in them to have you flattened because there’s something so raw and pure and brave and true.

“If you don’t let your intellect get in the way and you are brave enough to let it affect you in a more visceral, holistic way, to look at humanity squirming in darkness for 55 minutes, it is utterly arresting.”

Not I/Footfalls/Rockaby runs at The Duchess Theatre until February 15. 0844 482 9672. www.nimaxtheatre.com. Hampstead actor Peter Egan brings Samuel Beckett’s “moving and powerful” 1946 novel First Love to life in a performance in aid of Burgh House on February 12. Tickets cost £12. Bookings www.burghhouse.org.uk. 020 7431 0144.